This invention relates to heterodyne receivers and, particularly, to wideband, heterodyne receivers which tune relatively close to zero frequency.
Heterodyne receivers capable of tuning from very low frequencies to much higher frequencies are widely used in communications equipment as well as test and instrumentation equipment such as network and signal analyzers. The band of interest in frequency domain analysis of amplifiers, filters, mixers and many other types of linear and nonlinear networks and systems, as well as in analysis of mechanical vibration, often includes frequency components which are many orders of magnitude apart, sometimes as widely separated as sub-audible and microwave frequencies. Tuning over such wide bands of interest, whether discrete or swept, presents difficulties not encountered in other tuning situations.
Conventional heterodyne receivers include an RF amplifier and a local oscillator coupled to respective inputs of a mixer which in turn is connected to an IF stage. Such receivers are tuned to a desired input signal frequency by setting the local oscillator frequency above or below the first IF frequency by the amount of the desired input frequency such that the difference or sum frequency component produced by the mixer is at the IF frequency. Whether the sum of difference frequency component from the mixer is desired, for tuning close to zero frequency such conventional systems require that the local oscillator be set to a frequency which is close to the IF frequency. Since a mixer produces a number of frequency components including both the signal and local oscillator frequencies as well as the sum and difference thereof, when a conventional receiver is tuned close to zero frequency, a signal at the local oscillator frequency passes through the IF stage to the detector where it cannot be discriminated from the IF signal itself. Moreover, if the amplitude of the local oscillator signal passing through the mixer is excessively high, the IF circuits may saturate as a result of the unwanted signal, causing spurious signals and loss of gain.
One way to reduce the feedthrough problem just described is to use a first IF filter with a very narrow passband. Precision parts are required for high-Q filters, and the cost of such filters is accordingly high. Also, the bandwidth of the filter may be required to be wider for other reasons, such as ease of signal detection during coarse tuning.
An alternative approach is to avoid the problem by switching both the local oscillator (L.O.) and first IF frequencies to much lower frequencies when tuning close to zero frequency. For example, in a system where the first IF frequency is fixed at 100 MHz, when the system is tuned to receive a signal close to zero frequency, e.g., 100 KHz, the L.O. would have to be tuned to 100.1 MHz to tune in the 100 KHz signal, and the feedthrough at 100.1 MHz would be well within the passband of the 100 MHz IF amp. Instead, the IF frequency might be switched down to 1 MHz and the L.O. switched down in frequency accordingly. The L.O. would then be tuned to 1.1 MHz to receive the 100 KHz signal, proportionally farther away from the first IF frequency of 1 MHz than 100.1 MHz is from 100.0 MHz. With this approach, the L.O. frequency feedthrough can be adequately suppressed in a conventional IF filter. However, additional circuitry including switching circuitry is necessary with this approach.